Strainer for plastic material



jam, 23 151 A. N. GRAY STRAINER FOR PLASTIC MATERIAL Filed Aug. 50, 1946 Patented Jan. 23, 1951 2,539,128 v STRAINER FOR PLASTIC MATERIAL Alvin N. Gray, Joppa, Md., assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application August 30, 1946, Serial No. 693,924

4 2 Claims.

This invention relates to strainers. and more particularly to strainers for straining oversizezl particles from plastic material.

In the manufacture of products from plastic material, such as vulcanizable compounds of rubber or synthetic rubber-like materials, it is sometimes desirable to strain the material. Some strainers include backing plates which have passages extending therethrough, and the backing plates work the material being forced throrgh the passages therein to aid in plastlcizing the material. The size of the passages in such a backing plate must be sufilciently small to work the material sufiiciently to heat the material to the desired temperature and for the plate to have sufficient strength, but must be sufficiently large to permit the required rate of flow of the material therethrough. If screens are placed over the backing plate to strain out any oversizecl particles in the material, the efiective area of the screens must be sufficient to permit the desired quantity of material to flow therethrough even when the screens become partially clogged from accumulated oversized particles strained from the material. In the past, no strainer including a backin plate and a screen has been known which provided suiliciently small openings in the backing plate to work the material and permit the plate to retain sufficient strength and yet retain sufficient effective screening area of the screen so that sufficient material would be passed through the screen and the backing plate if the screen became partially clogged.

An object of the invention is to provide new and improved strainers.

A further object of the invention is to provide new and improved strainers having provided both suflicient flow capacity and sufiicient strength.

A strainer illustrative of the invention includes a backing plate and a screen. The screen is supported by the backing plate, and both are positioned at the end of an extrusion passage through which a stock screw forces material. The backing plate is provided with a plurality of passage", therethrough which are enlarged at the portions thereof adjacent to the screen so that the effective straining area of the screen is larger than the area of the passages. Hence, when the screen becomes partially clogged from oversized particles strained from the material, there is sufficient unclogged area of the screen to permit the necessary rate of flow of the material.

A complete understanding of the invention may be obtained from the following detailed description of strainers forming specific embodiments thereof, when read in conjunction with the appended drawings, in which:

Fig. 1 is a fragmentary, horizontal section of an extrusion apparatus including a strainer forming one embodiment of the invention;

Fig. 2 is a fragmentary, end viewof a portion of the strainer shown in Fig. 1;

Fig. 3 is a fragmentary, end view of a strainer forming an alternative embodiment of the invention, and r Fig. 4 is an enlarged, horizontal section taken along line 4-4 of Fig. 3.

Referring now in detail to the drawings, there is shown in Fig. 1 a continuous extrusion apparatus provided with a strainer 8 forming one embodiment of the invention. The continuous extrusion apparatus is designed to apply a covering of plastic material, such as a vulcanizable compound including rubber or a synthetic rubberlike material, upon a filamentary conductor 9, which may be bare or covered with a textile 01' plastic covering. This apparatus includes an extrusion cylinder l having a cylindrical extrusion bore H formed therein in which an elongated stock screw I2 is rotatably mounted. The stock screw is rotated by suitable driving means, such as an electric motor and a gear train (not shown), to force the plastic'material through an extruding head l4 under high pressure.

The extruding head [4 includes a body mem ber l having a tapered opening I6, which forms a continuation of the extrusion bore l l and communicates with a bore formed in the extrudin head transversely of the tapered opening 16. An annular die holder 2| is positioned in the ex t end of the bore 20, and has a counter-bore 22 formed therein in which a formin die 23 is iii! mounted. The conductor 9 is advanced toward the right, as viewed in Fig. l, by suitable coreadvancing means, such as a capstan (not shown) through a core tube holder 25 and a core tube 26.

which is held by the core tube holder. The core tube holder is provided with an inclined concave surface 21 for deflecting the plastic material toward the die 23. An aligning plate 3| maintains the die holder 2| and the core tube holder 25, and thereby the die 23 and the core tube 26, cantered relative to each other in the bore 29 in the extruding head l4, and a retaining nut 32 holds these elements in place in the extruding head.

The extrusion cylinder IU of the continuous extrusion apparatus is provided with a counterbore and an annular shoulder 36. A fine straining screen 40, which should have a mesh of a size from about to about abuts the shoulder 58 at the outer portion of the screen and a heavy, woven backin screen 4|, which should have a mesh of a size from about It to about Ill, abuts the straining screen 40. A backing plate 42 presses the fine screen 40 through the backing screen 42 against the shoulder 35, and the backing plate 42 includes an annular projection 45. The annular projection fits into a socket 46 formed in the extrusion head, which holds the backing plate 42 in position in the counterbore 35 formed in the extrusion cylinder ID.

The backing plate 42 is provided with a plurality of cylindrical passages 41-41 through which the plastic material is forced by the stock screw I2, and which work the plastic material sufllciently to plasticize it. The passages 41-41 are arranged in radial rows, of which radial rows 50 and (Fig. 2) are shown, rows 52-52, which are parallel to the radial row 50, and rows 53-53, which are parallel to the radial row 5|. Grooves 55-55 extending along the rows 50-54, 5I-5I, 52-52 and 53-52 are formed in the backing plate 42, and are separated from each other by ribs 56-55. The grooves 55-55 are relatively shallow, and are substantially rectilinear in cross-section.

The ribs 56-56 engage the backing screen 4I and help support it against the high extrusion pressure of the plastic material being forced through the apparatus by the stock screw II. However, the portions of the screens 40 and 4| between the ribs 56-55 sag gradually toward the portions thereof intermediate of the ribs, which intermediate portions of the screen 4| are supported by the central portions of the bottoms of the shallow grooves 55-55. Hence, the span between supporting points of the backing plate 42 is substantially shorter than the distance between adjacent ribs so that the screens 40 and 4| are not subjected to so severe strains as they would be if they were supported by only the ribs 56-56. The portions of the grooves 55-55 between the central portions of the grooves 55-55 and the ribs 55-56 are spaced from the backing screen 4| so that the plastic material may flow through the screens 40 and 4| throughout substantially the entire portions of their areas which are facing the grooves 55-55. Hence, the effective screening area of the screen 4| is much greater than the total transverse area of the passages 41-41, and even if portions of the screen 40 become clogged from oversized particles strained from plastic material, sufficient screening area is provided so that the screen 40 need not be removed from the apparatus for cleaning except at very long intervals. In contrast, if the grooves 55-55 were not provided, the screen 40 would have to be removed from the extrusion apparatus for cleaning at much shorter intervals than is necessary where the grooves 55-55 are provided.

In the operation of the apparatus described hereinabove, the plastic material is forced through the extrusion bore II by the stock screw I2, and is forced through the straining screen 40, which screens any oversized particles therefrom, and the backin screen 4|, which supports the straining screen 40. The material is forced from the backing screen 4| through the grooves 55-55 into the passages 41-41 in the backing plate 42, which work the material sufliciently that, with the working or the ma terial by the stock screw I2 and the screens 40 i 4 and 4|, the material is plasticized when it is formed into a covering upon the conductor 5.

The material extruded upon the conductor 5 is completely strained and plasticized so that a very high quality covering is iormed therefrom. The grooves -55 increase the effective screening area of the screen 40 to such an extent that the straining screen 40 need be removed from the apparatus only at infrequent intervals, and the straining screen 44 even though partially clogged does not greatly slow the rate of extrusion of the plastic material. The grooves 55-55 may be formed in the backing plate 42 by milling or other similar operations, which require relatively little eilort.

A strainer I08 (Figs. 3 and 4) forming an altenative embodiment of the invention includes a backing plate I42, which may be used in place of the backing plate 42 (Fig. 2) in the continuous extrusion apparatus. The strainer I is identical with the strainer 5 except that the backing plate I42 of the strainer III does not include the grooves 55-55 and the ribs 55-56 oi. the backing plate 42. The backing plate I42 is provided with passages 1-HT, and also is provided with shallow counterbores I 55-l 55, which have a substantially larger diameter than the diameter of the passages I4I-I4l. Hence, the area of the strainin screen 44 (Fig. 1) through which material may flow into the counterbores I55-I55 of the passages I4'I-I4'I is much larger than the total cross-sectional area of the passages I4l-I4I so that the straining screen may become partially clogged without substantially affecting the flow of the material through'the Dlasticizing passages I4I-I4l.

The counterbores I55-I55 have sufliciently large diameters and are sufliciently shallow that a coarse backing screen I corresponding to the screen 4| sags into contact with inner edges I 58-I 58 or the bottoms of the counterbores. The edges I58-I58 and a face I55 of the backing plate I42 strongly support the backing screen I4| at relatively closely spaced portions thereof. The backing screen I is woven so that the material may flow facewise therethrough from the counterbores I55-I55 to the passages I41- I41. Hence, even though the backing screen MI is in contact with the inner portions of the bottoms of the counterbores, the material flows to the passages I4I-I4I through all oi. each of the portions of the screen I 40 facing the counterbores I55-I55. That is, the eiIective straining portion of the area of each of the portions of the screen I40 directly over the counterbores and the passages I4I-I 41 is substantially the whole of that area while the spans of the unsupported portions of the screen I are relatively short.

The strengths of the backing plates 42 and I42 are sufilciently high to withstand the high extrusion pressures of the material forced therethrough by the stock screw i2 and contact the screens 4| and I at sumciently close points thereof so that the screens 4| and I are protected against unduestress, but contact these screens at small enough areas that the screens 40 and I40 have more than suflicient efifective screening areas to permit the required rate of flow oi! plastic material therethrough even though the straining screens 40 and I40 may be partially clogged by oversized particles strained thereby.

What is claimed is:

1. A strainer for a conduit, which comprises a fine screen extending transversely across an exit of a conduit, a coarse screen abutting the fine screen for supporting the fine screen, and a back- 1 ing plate provided with a plurality of passages positioned in rows therein, said backing plate being in contact with the coarse screen to support the screens against the pressure of plastic material being forced through the conduit, said backing plate also being provided with a plurality of shallow grooves extending along the rows oi the passages and having a plurality of ribs positioned between the grooves for supporting the coarse screen, said grooves being shallow enough to support the coarse screen along the central portions of the bottoms thereof and deep enough to permit flow of material along the grooves, whereby the efiective straining area of the screen is high.

2. A strainer, which comprises a fine straining screen extending across the exit of an extrusion bore, a coarse woven, backing screen positioned against the fine screen, and a backing plate positioned against the backing screen for supporting the backing screen On one face thereof, said backing plate being provided with a plurality of bores therein, said backing plate also being provided with shallow counterbores in the face thereof on which the backing screen is supported to support the woven backing screen at the inner portions of the bottoms of the counterbores, said counterbores permitting flow of material therethrough so that the eflective straining area of the fine screen is high.

ALVIN N. GRAY. 10

REFERENCES CITED The following references are of record in the file of this patent:

15 UNITED STATES PATENTS Number Name 'Date 1,446,187 Kessler Feb. 20, 1923 1,732,618 Royle Oct. 22, 1929 1,931,872 Lodge Oct. 24, 1933 my 2,135,325 Burt et al Nov. 1, 1938 2,249,063 Swem July 15, 1941 2,261,977

Deutsch NOV. 11, 1941 

